1. Field of the Invention
This invention relates to thermoplastic compositions based on polyphenylene ethers, impact resistant styrene polymers, and pulverulent, filler-containing rubber. This invention also relates to a method for the manufacture of these compositions.
2. Description of the Prior Art
Thermoplastic compositions suitable for the production of molded parts and comprised of modified styrene polymers and polyphenylene ethers (PPE) were first described in German Patent No. 1,694,257. German Patent No. 2,211,005 discloses that preferably, the fraction of the finely distributed elastomeric gel phase in the rubber-modified polystyrene should be greater than 22 wt. %.
The thermoplastic compositions disclosed in German Patent No. 2,119,301 are comprised of polyphenylene ethers with a rubber-modified polystyrene and/or a polystyrene and a rubber.
For compositions in which the mean particle diameter of the dispersed elastomeric phase is less than a maximum value of about 2 microns, higher impact resistances are said to be attainable.
The molding compositions based on polyphenylene ethers and polybutadiene, described in German Patent No. 1,694,255, are of lesser industrial importance. The reason for this may be that the two components are not infinitely miscible, in contrast to the case in which polystyrene is used.
It is known to add fillers, e.g. metals, titanium dioxide, asbestos, glass fibers, calcium silicate, or aluminum silicate (DE-OSS Nos. 27 13 462, 27 52 383, and 28 40 111) to polymer mixtures in order to improve their physical properties. By the addition of aluminum flake, carbon fibers, and carbon black, an electrical shield effect can be attained in the molded items produced from these polymer mixtures.
The filler-containing thermoplastic masses are produced by thoroughly mixing the components, namely, the polyphenylene ether, the styrene resin, and the filler, extruding the molten premixture to form a cord, and comminuting the latter to form a granulate (see DE-OS No. 27 13 462). Among the polymer mixtures based on polyphenylene ethers those with high impact strength and high distortion temperature are especially important.
In principle, mixtures based on polyphenylene ethers and impact-resistant styrene polymers (also called "high impact styrene polymers" ("HISP")) may be produced with various contents of rubber. However, this approach is costly because a large number of different types of polystyrene must be kept available so that adjustments can be made to achieve desired qualities in each given case. It would be simpler if various types of products could be produced by appropriate mixing of just a few starting components.
One possible approach is to start with an impact-resistant styrene polymer having a high percentage of rubber mixed or incorporated therein, and then to thin this material to the degree desired with polystyrene.
It would be still more advantageous if one could start with an impact-resistant styrene polymer with low rubber content, since this is not only more readily available but also much less expensive.
It is technically possible to produce polystyrene and then to incorporate (e.g., mix) the rubber into the polystyrene in a subsequent operation. But the rubber, which is ordinarily employed in the form of bales is difficult to handle. Moreover, for a given content of rubber, molded parts produced from such polymer mixtures have a lower measured impact resistance than is attained by impact-resistant styrene polymers (see German Patent No. 2,119,301, col. 2 lines 51 ff.). Thus the purpose of adding the rubber is defeated.
Accordingly, there is a strong need for a method permitting the advantageous addition of rubber, in pulverulent form and containing various amounts of fillers, to the subject synthetic plastic base compositions.
No examples of this are known from the literature, i.e., no examples of practicable compositions produced by addition of such rubber mixtures to polyphenylene ethers and impact-resistant styrene polymers are known. All that is known is that pulverulent, filler-containing rubbers which contain certain percentages of thermoplastics (such as polyvinyl chloride, polystyrene, or polymethyl methacrylate), may be prepared. See German Patent No. 1,694,918. However, this patent publication does not give one skilled in the art any encouragement, stimulus, or clue as to the preparation of compositions of this type which are in fact practicable--a fortiori since the method described there has been a commercial failure.
Research conducted by the present Applicants has in fact shown that natural-rubber-based rubber compositions based on mixtures with polyphenylene ethers and impact-resistant styrene polymers lead to molding compounds with sharply reduced impact resistance (see Comparison Examples B and C, infra). Unsatisfactory results were also obtained with polybutadiene-based rubbers in pulverulent form. In view of these results it appeared unpromising to attempt to develop polymer compositions comprised of polyphenylene ethers, styrene resins and pulverulent, filler-containing rubber. Accordingly the strong need for a method permitting the advantageous addition of rubber, in pulverulent form and containing various amounts of fillers, to the subject synthetic plastic base composition remained.